Chapter 5
GLAMORGAN AND GWENT-- SOUTHEAST WALES.
Fig. 2. Geologic map of the lower Usk valley and the Forest of Dean.
The Forest of Dean is a plateau eroded across Carboniferous rocks. The bedrock structure of the main plateau is a shallow basin, like a saucer with beds dipping in towards the center. A coarse and very resistant conglomerate at the top of the Old Red Sandstone rims the edge of the saucer. This conglomerate also occurs on the Sugarloaf, north of Abergavenny, and the Black Mountains and Brecon Beacons to the west. Within the saucer, Carboniferous Limestone and the Coal Measures lie on top of the conglomerate. Although outcrops are rare in the Forest of Dean, many abandoned mines betray the presence of the Coal Measures.
Micheldean (366 219)-- Monmouth (351 213). 10 miles (16 km)
Very few outcrops exist along the A4136, the main road through the northern part of the Forest of Dean. Bedrock appears at Staunton (355 212), east of Monmouth, where the road drops sharply into the Wye Valley. A large road cutting at the west end of the village (Fig. 3), shows Old Red Sandstone conglomerate crowded with white pebbles of quartz. The rest of the rock is red sandstone and mudstone, typical of the Old Red Sandstone. The beds dip down to the east beneath the Carboniferous rocks above them. This is the hard conglomerate rim-rock that supports the scarp around the Forest of Dean. As the road descends into the Wye Valley, it drops down the sequence into softer sandstone of the Old Red Sandstone that shows only as a distinctive red soil.
Fig. 3. Old
Red Sandstone
conglomerate. Pebbles of quartz, up to three centimetres, in a red
sandstone. View is a metre across.
The Wye Valley
Monmouth (351 213) -- Chepstow (353 193). 16 miles (26 km)
The River Wye forms part
of the boundary between England and Wales.
It cuts deeply into the plateau of the Forest of Dean,
meandering across the bedrock structure (Fig. 4) as it wends its way to
join
the River Severn, ignoring the differences in the rock types. The river
developed its sinuous course when it flowed across a nearly featureless
plain
near sea level. When the plain rose to its present level, the river cut
straight down into its bed, through whatever rock was underneath.
Fig. 4. Symonds Yat (356 216), River Wye. The river Wye cuts deeply into a plateau eroded across Carboniferous Limestone.
The A466 runs along the Wye Valley between Monmouth and Chepstow. Along most of the route, the road follows the meanders of the River Wye through the Old Red Sandstone. The relatively soft rocks erode to a fairly broad valley with few outcrops, but a few ribs of harder sandstone line the valley walls above the road.
South of Tintern Abbey (354 201), the valley narrows as the river cuts into the younger Carboniferous Limestone. Wyndcliff (353 197), a nature walk about halfway between Tintern Abbey and Chepstow, climbs up high cliffs of these limestones. Vertical joints are common in the limestone; the rocks fall away from these joints and keep the cliffs (Fig. 5) from being worn into more moderate slopes.
Fig. 5. Wyndcliff on the A466, north of Chepstow. Well bedded Carboniferous Limestone with vertical joints.
Monmouth (351 213) to Newport (332 187). 25 miles (40 km)
The A40 follows a broad valley between Monmouth to Raglan (342 208), past a few road cuttings of conglomerate of the Old Red Sandstone.
The A449 between Raglan and Newport follows a wide valley eroded in Old Red Sandstone, between the South Wales Coalfield on the west and the Forest of Dean on the east. The valley runs down the middle of a broad arch, an anticline, which separates the synclinal troughs of the South Wales Coalfield and Forest of Dean. This anticline brings older, Silurian, sandstone and siltstone to the surface around Usk, but the only place to see outcrops are in streambeds on private land. The soil on the Silurian rocks is brown, while it is red on the Devonian Old Red Sandstone. Just north of the junction with the M4 the road drops onto the coastal plain of the Bristol Channel.